There are a variety of hydrocarbon extraction processes for producing hydrocarbon-containing fluids. Typically, the produced fluids are emulsions including both hydrocarbons and "hard water," where the phrase "hard water" denotes water having impurities that form insoluble precipitates known as "scale" when the hard water is heated. It is desirable to separate the produced, hydrocarbon-containing fluids into a predominantly liquid hydrocarbon portion, a predominantly liquid water portion, and a gaseous portion. The predominantly liquid water portion will typically include hard water.
For many applications, it is desirable to "soften" the liquid water portion by decreasing substantially the concentration of impurities (generally, these are divalent metal ions) in the hard water which can form insoluble precipitates. One suitable technique for softening hard water is the thermal softening process described in U.S. Pat. No. 4,518,505, issued May 21, 1985 to G. B. Lim and A. R. Konak, and assigned to Exxon Production Research Company. A system for processing a fluid in accordance with the thermal softening process disclosed in such patent will be referred to herein as a "thermal softening unit."
For many applications, it is desirable not only to soften the liquid water portion of the produced fluid but to reduce substantially the concentration of hydrocarbons in the liquid water portion. The concentration of hydrocarbons in a fluid will hereinafter be referred to as the "oil concentration" of the fluid and will be quantified in units of parts per million ("ppm").
In one important example, a bitumen-containing emulsion is extracted from a tar sand formation using a conventional cyclic steam stimulation process. Steam, typically having quality of about 80%, is injected into the formation to mobilize bitumen which is subsequently produced with the steam condensate as emulsion. This emulsion typically contains 3 to 4 volumes of water for each volume of bitumen and its temperature is approximately 140.degree. C. once the steady state conditions are established. The emulsion is typically first cooled to around 125.degree. C. to recover some of its heat and raise boiler feedwater temperature from approximately 100.degree. C. to approximately 133.degree. C. It is then separated in an inlet separator into its gas, oil and water components at about 125.degree. C. The inlet separator thus acts as an initial separator. Also the inlet separator takes up the production surges that frequently occur in this type of steam stimulation operation. Most of the free water (up to 75% or more) is separated from the hydrocarbon and gas components by the inlet separator. The bitumen along with the remaining liquid water is separated into approximately equal streams for further cooling and treatment at two or more free and emulsified water removal units. Bitumen with a bottom sediment and water specification (BS & W specification) of 1/2% or less emerges from these units for blending with diluent prior to pipelining. A water stream emerges from each free water removal unit at around 110.degree. C. and is further cooled and combined with the water stream emerging from the inlet separator, and the combined stream is further cooled prior to storage and further oil separation at atmospheric pressure. The oil content of the combined water stream is typically approximately 1% to 5%. Some of the oil is skimmed off in a skim tank and is recycled. The water, at 80.degree.-90.degree. C., is then further treated by induced gas floatation and coalescing filters to reduce its oil concentration to 10 ppm or less before the deoiled water is softened in a hot lime treater (HLT) and then filtered by anthracite filters. Remaining traces of hardness are removed in a set of ion exchange units and the deoiled, softened water is then stored in a boiler feedwater tank. Deoiled, softened water from this tank is heat exchanged with the inlet production to recover some of the heat as mentioned earlier, and then is fed to a boiler for steam generation purposes. Also make up fresh water used in the HLT is heated up using a portion of the heat recovered from the inlet production.
It is emphasized that the conventional system described above, and similar conventional systems, employ heat exchangers to extract heat from the produced fluid either before the fluid is separated into its components or after various components have been separated from it (or both before and after separation) for the purpose of transferring the extracted heat to boiler feed water while reducing the fluid temperature to about 80.degree.-90.degree. C., to facilitate deoiling at about 80.degree.-90.degree. C. and atmospheric pressure.
The conventional system described above has several drawbacks. First, the produced fluid, or the predominantly liquid water component thereof, needs to be cooled down and heated up again to deoil and soften it. Second, not all the heat can be recovered and used in the system and excess heat is dissipated to atmosphere. Third, the steam generation facilities are coupled with bitumen production facilities through the heat exchanger that heats up the boiler feedwater and cools down the inlet production. Therefore when one facility is upset, the other is affected. Fourth, the water reuse system is complex and entails large capital and operating costs.